1. Field of the Invention
This invention relates to a phase servo circuit. More particularly, the present invention relates to a phase servo circuit for controlling a rotatable magnetic head assembly in a video signal recording/reproducing apparatus having an automatic tape editing function.
2. Description of the Prior Art
In a helical scan type of video tape recorder (VTR) having two-heads, one head, one head with an auxiliary head, an automatic tape editing function is required when recording a second video signal to to be inserted on a portion of a magnetic tape on which a first video signal has been previously recorded or when joining the second signal on a tape at the end of a previously recorded first signal. Alternatively, a separate editing VTR can be used for such editing. Generally, control and synchronizing signals in the first and second video signal which are recorded on the magnetic tape have their phase adjusted with respect to each other before and after the respective editing point so that the final reproduced picture is fully synchronized.
Generally, there are two types of editing, that is, "assemble" and "insert", as illustrated in FIGS. 1A and 1B, which show track patterns on a magnetic tape. In the "assemble" mode, after one cut of signal A of a recording is finished, a next cut of signal B is recorded so as to join it at the end of the first cut of signal A. A fixed full erase head is used in this mode to erase the entire recorded signal A including control signals (CTL), and then a new video signal B is recorded together with its control signals CTL on magnetic tape 1. It should be appreciated that parts of the recorded signal A on some tracks are erased, which parts are not desired to be erased, if the full erase head is immediately operated. Thus, a fixing-erase head or rotational erase head is operated for a predetermined time to erase certain video tracks of signal A, as shown by dotted lines in FIG. 1A. The full erase head is then operated after a point P, that is, a point where the full erase head does not catch any parts of the tracks of signal A.
For the "insert" mode, a B-video signal is inserted in, rather than after, the recorded A-video signal. In this mode, the B-video signal (dot-dashed lines) is recorded after a portion of the A-video signal (solid lines) is erased by the flying erase head, with the exception of the CTL signals, as shown in FIG. 1B.
In the above "assemble" and "insert" editing modes, a rotary head-phase servo system of the editing VTR is locked on the basis of the vertical synchronizing signals of the recording B-video signal, while the recorded A-video signal is reproduced for carrying out a "capstan tracking servo" operation which controls tape transportation on the basis of the reproduced CTL signals and rotational phase detecting pulses (PG pulse) of the rotary drum assembly. Therefore, the phase relation between the vertical sync signal of the recording B-video signal and the reproduced CTL signal of the recorded A-video signal is adjusted in the editing VTR before the tape arrives at the editing point on the tape. After arriving at the editing point, the editing VTR is changed over into its recording mode for recording the B-video signal, and the capstan servo system thereof only carries out a constant speed servo operation, in the "assemble" mode. On the other hand, in the "insert" mode, only the head drum servo system of the editing VTR is changed over to the recording mode and the capstan servo system is kept in its reproducing mode whereby the tracking servo operation is carried out on the basis of the reproduced CTL signal.
In the above manner, the phase relation between the sync signals in the A-video signal and the B-video signal are adjusted with respect to each other before and after the editing point, under normal conditions. However, if there is no satisfactory interchangeability between the playback VTR for obtaining the B-video signal and the editing VTR, or if the accuracy of the rotary head assembly and tape transportation assembly changes with the passage of time, phase agreement between the edited signals is not completely achieved.
For example, as shown in FIG. 2A, if there is some phase difference .DELTA.V between the vertical sync signals V of the recording B-video signal and those of the recorded A-video signal, the reproduced picture is vertically shifted at the editing point when the tape is played-back. Moreover, if there is some phase difference .DELTA.H (FIG. 2B) between the horizontal sync signals H in the A- and B-video signals, horizontal oscillation of the monitor TV is pulled out and the reproduced picture drifts. In other words, "H-step out" occurs on the picture.